[gdb/testsuite] Fix exec_is_pie with gold linker

[gdb/testsuite] Fix exec_is_pie with gold linker

When running test-case gdb.base/break-interp.exp with target board gold, we
run into:
...
gdb compile failed, pie failed to generate PIE executable
...

The problem is that the proc exec_is_pie uses the PIE flag in the readelf -d
output, which doesn't seem to be set by the gold linker.

Instead, use the "Type" field in the readelf -h output.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-05-25  Tom de Vries  <tdevries@suse.de>

	PR testsuite/26031
	* lib/gdb.exp (exec_is_pie): Test readelf -h output.