aarch64: libgcc: Cleanup warnings in lse.S

Since
  Commit c608ada2
  Author:     Zac Walker <zacwalker@microsoft.com>
  CommitDate: 2024-01-23 15:32:30 +0000

  Ifdef `.hidden`, `.type`, and `.size` pseudo-ops for `aarch64-w64-mingw32` target

lse.S includes aarch64-asm.h, leading to a conflicting definition of macro 'L':
- in lse.S it expands to either '' or 'L'
- in aarch64-asm.h it is used to generate .L ## label

lse.S does not use the second, so this patch just undefines L after
the inclusion of aarch64-asm.h.

libgcc/
	* config/aarch64/lse.S: Undefine L() macro.