[gdb/tdep] Simplify ARM_LINUX_JB_PC_EABI

[gdb/tdep] Simplify ARM_LINUX_JB_PC_EABI

In commit 1a7d840a216 ("[gdb/tdep] Fix ARM_LINUX_JB_PC_EABI"), in absense of
osabi settings for newlib and uclibc for arm, I chose a best-effort approach
using ifdefs.

Post-commit review [1] pointed out that this may be causing more problems than
it's worth.

Fix this by removing the ifdefs and simply defining ARM_LINUX_JB_PC_EABI to 1.

Rebuild on x86_64-linux with --enable-targets=all.

Fixes: 1a7d840a216 ("[gdb/tdep] Fix ARM_LINUX_JB_PC_EABI")
[1] https://sourceware.org/pipermail/gdb-patches/2024-June/209779.html