fix(build): allow warnings when using lld

fix(build): allow warnings when using lld

After https://reviews.llvm.org/D118840, ld.lld reports
below section type mismatch warnings:

  LD      trusted-firmware-a/build/qemu/release/bl1/bl1.elf
ld.lld: error: section type mismatch for base_xlat_table
>>> trusted-firmware-a/build/qemu/release/bl1/xlat_tables_context.o:
    (base_xlat_table): SHT_PROGBITS
>>> output section .bss: SHT_NOBITS

ld.lld: error: section type mismatch for xlat_table
>>> trusted-firmware-a/build/qemu/release/bl1/xlat_tables_context.o:
    (xlat_table): SHT_PROGBITS
>>> output section xlat_table: SHT_NOBITS

So allow warnings to make build pass.

Change-Id: I70503ad03b3be7a18ffe29ef6f2127c3f4d540f1
Signed-off-by: Yabin Cui <yabinc@google.com>