gcc: xtensa: don't force PIC for uclinux target

gcc: xtensa: don't force PIC for uclinux target

xtensa-uclinux uses bFLT executable file format that cannot relocate
fields representing offsets from data to code. C++ objects built as PIC
use offsets to encode FDE structures. As a result C++ exception handling
doesn't work correctly on xtensa-uclinux. Don't use PIC by default on
xtensa-uclinux.

gcc/
2018-11-07  Max Filippov  <jcmvbkbc@gmail.com>

	Backport from mainline
	2018-11-05  Max Filippov  <jcmvbkbc@gmail.com>

	* config/xtensa/uclinux.h (XTENSA_ALWAYS_PIC): Change to 0.

From-SVN: r265889