kallsyms: ignore all local labels prefixed by '.L'

The llvm compiler can generate lots of local labels ('.LBB', '.Ltmpxxx',
'.L__unnamed_xx', etc.). These symbols usually are useless for debugging.
And they might overlap with handwritten symbols.

Before this change, a dumpstack shows a local symbol for epc:
[    0.040341][    T0] Hardware name: riscv-virtio,qemu (DT)
[    0.040376][    T0] epc : .LBB6_14+0x22/0x6a
[    0.040452][    T0]  ra : restore_all+0x12/0x6e

The simple solution is that we can ignore all local labels prefixed by '.L'.
For handwritten symbols which need to be preserved should drop the '.L'
prefix.

After this change, the C defined symbol is shown so we can locate the
problematical code immediately:
[    0.035795][    T0] Hardware name: riscv-virtio,qemu (DT)
[    0.036332][    T0] epc : trace_hardirqs_on+0x54/0x13c
[    0.036567][    T0]  ra : restore_all+0x12/0x6e

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>