Add a minimal UEFI target

[sparques]

Summary

This PR adds a minimal uefi-amd64 target to TinyGo.

The intent is to land the smallest useful slice of UEFI support first:

• target definition for uefi-amd64
• PE/COFF linking support through ld.lld
• minimal UEFI runtime entry/exit path
• minimal firmware bindings needed for startup, console output, timing, and heap allocation
• a tiny clean-exit example for validation

This PR does not try to upstream the larger protocol surface from the previous UEFI branch. It is intentionally limited
to getting a basic UEFI application built and run as a TinyGo baremetal target.

What this adds

• targets/uefi-amd64.json
• generic target-controlled linker flavor selection, so a target can request PE/COFF-style ld.lld handling without a UEFI-
  specific builder special case
• minimal machine/uefi support for:
  • system table access
  • boot services calls
  • text output
  • simple timing support
• minimal UEFI runtime support for:
  • efi_main entry
  • heap allocation through AllocatePages
  • clean return from a do-nothing application
• minimal amd64 support code needed by this target
• examples/uefi-exit as a simple regression-style smoke example

What this does not add

This PR intentionally leaves out higher-level UEFI features, including:

• filesystem protocols
• graphics output
• networking
• serial I/O
• environment variables / argument handling
• broader protocol coverage

Those can be added in follow-up PRs once the target itself is established.

Design notes

TinyGo baremetal targets commonly use goos: "linux" for runtime selection, and this target follows that existing
convention.

The unusual part of UEFI is not the runtime model, but the output format: the target must produce a valid PE/COFF image. The linker changes here keep the working ld.lld-based approach, but move it behind a generic target property instead of a UEFI-specific special case in the builder.

The runtime in this PR is intentionally minimal:

• gc=leaking
• scheduler=none

That keeps the initial target small and reduces the number of moving pieces needed for first upstream support.

Testing

Built successfully with:

./build/tinygo build -size short -o /tmp/tinygo-uefi-empty.exe -target=uefi-amd64 examples/empty
./build/tinygo build -size short -o /tmp/tinygo-uefi-exit.exe -target=uefi-amd64 ./examples/uefi-exit

Observed behavior:

- firmware accepts the generated PE/COFF image
- examples/empty runs as expected
- examples/uefi-exit cleanly returns to firmware (this is more of a challenge than it sounds)