Add esp32c3-qemu exec elf support

[YinhuaChen-vivo]

Summary

Add fixed-address RV32 ET_EXEC loading support for ESP32-C3 while keeping the
existing position-independent ET_DYN flow intact.

The loader now selects the loading strategy from both the ELF type and the
mapper mode:

                         +------------------------------+
                         |          load_elf()          |
                         +---------------+--------------+
                                         |
                                  parse ELF header
                                         |
                     +-------------------+-------------------+
                     |                                       |
              ET_DYN + allocated                    ET_EXEC + fixed
                     |                                       |
          allocate backing storage                validate RV32 ELF
          copy and zero segments                   validate ranges/flags
          apply RELATIVE relocations               copy and zero segments
          translate entry point                    fence.i and use vaddr
                     |                                       |
                     +-------------------+-------------------+
                                         |
                                  executable entry

            Any ELF/mapper mismatch or failed validation is rejected.

What changed

• Add a fixed mapping mode to MemoryMapper, backed by caller-authorized
  MemoryRegions and explicit read/write/execute permissions.
• Support static, fixed-address, 32-bit little-endian RISC-V ET_EXEC images.
  Loadable segments are preflighted before any memory is written, including
  input bounds, address overflow, alignment, permissions, and executable-entry
  checks.
• Reject dynamic segments and runtime relocations for ET_EXEC, and execute
  fence.i after installing code on RISC-V so the instruction stream observes
  the newly loaded image.
• Harden the existing ET_DYN path with checked ELF integer conversions and
  range arithmetic, explicit BSS zeroing, load-bias-aware relative
  relocations, aligned writes, and rejection of unsupported dynamic
  relocations.
• Add a statically linked RV32 test application placed in ESP32-C3 RTC fast
  memory at 0x50000000. The fixture verifies initialized data, zeroed BSS,
  and execution through the ELF entry point.
• Select the ESP32 QEMU runner for boards using the ESP32 loader while
  preserving the existing generic QEMU path and PIC ELF tests for other
  targets.

Validation

The ESP32-C3 integration suite covers:

• loading and executing the fixed-address ELF successfully;
• initialized-data copying and BSS zeroing;
• rejecting an ET_EXEC image with an allocated mapper;
• rejecting a segment outside the authorized region before modifying memory;
• rejecting a non-executable destination region; and
• rejecting an entry point outside an executable load segment.

Existing non-ESP32 integration tests continue to exercise the PIC/ET_DYN
path and malformed ELF inputs.

Limitations

• Fixed-address execution currently accepts only 32-bit little-endian RISC-V
  ET_EXEC images.
• ET_EXEC images must be fully static: PT_DYNAMIC and runtime relocations
  are not supported.
• The caller remains responsible for reserving and mapping every advertised
  MemoryRegion for its full lifetime and for avoiding conflicting accesses.