Fix FAILs in compare-sections.exp

Fix FAILs in compare-sections.exp

compare-sections.exp has two cases that are not handled appropriately:
1) value read with msb set
2) error while patching that section

This patch adapts the "get value of read-only section" test to print
the value as an unsigned integer to fix 1) and test for the error
message to not set the written variable if read-only section cannot
be written to so as to solve 2).

2017-09-26  Thomas Preud'homme  <thomas.preudhomme@arm.com>
	    Pedro Alves  <palves@redhat.com>

gdb/testsuite/
	* gdb.base/compare-sections.exp (get value of read-only section): Read
	as unsigned value.
	(corrupt read-only section): Likewise and don't set written if patching
	failed.