Adding an undefined __c6xabi_DSBT_BASE via an EXTERN in the linker
script isn't ideal, as the symbol is not always needed.  This patch
adds the undefined symbol on encountering relocations where it is
implicitly referenced.

bfd/
	* elf32-tic6x.c (elf32_tic6x_check_relocs): Reference
	__c6xabi_DSBT_BASE explicitly for R_C6000_SBR_* relocs.
ld/
	* emulparams/elf32_tic6x_le.sh (EXECUTABLE_SYMBOLS): Don't define.