Bump GCC from 14.2 to 15.1

[TommyMurphyTM1234]

Replacement for this PR to split the bump of GCC and GDB into separate PRs:

• gdb: Bump to next major version. #1710

Standalone PR for bumping GDB to come.

[TommyMurphyTM1234]

OK - scratch that.
No matter what I do I keep getting this problem:

• gdb: Bump to next major version. #1710 (comment)

which means that I get this:

• https://github.com/riscv-collab/riscv-gnu-toolchain/pull/1712/files

Submodule gcc updated 38246 files

instead of what has happened in the past with similar bump PRs:

• https://github.com/riscv-collab/riscv-gnu-toolchain/pull/1341/files

Submodule gcc updated from 2ee5e4 to c891d8

I can't figure this out so am closing/abandoning this PR.

If somebody can explain the steps needed to bump a submodule such as GCC so that this issue doesn't arise that might help - thanks.