PR binutils/23460: Increase the max number of open files to 20

PR binutils/23460: Increase the max number of open files to 20

Increase the max number of open files to 20 for PR binutils/23460 test
which may have more than 16 file descriptors open:

lr-x------ 1 hjl hjl 64 Jan 16 16:49 0 -> /dev/null
l-wx------ 1 hjl hjl 64 Jan 16 16:49 1 -> pipe:[14151430]
lr-x------ 1 hjl hjl 64 Jan 16 16:49 10 -> /export/build/gnu/tools-build/x86_64-linux-toolchain/build-x86_64-linux/binutils/src/binutils-build/ld/tmpdir/pr23460c.o
lr-x------ 1 hjl hjl 64 Jan 16 16:49 11 -> /export/build/gnu/tools-build/x86_64-linux-toolchain/build-x86_64-linux/binutils/src/binutils-build/ld/tmpdir/pr23460d.o
lr-x------ 1 hjl hjl 64 Jan 16 16:49 12 -> /export/build/gnu/tools-build/x86_64-linux-toolchain/build-x86_64-linux/binutils/src/binutils-build/ld/tmpdir/pr23460e.o
lr-x------ 1 hjl hjl 64 Jan 16 16:49 13 -> /export/build/gnu/tools-build/x86_64-linux-toolchain/build-x86_64-linux/binutils/src/binutils-build/ld/tmpdir/pr23460f.o
lrwx------ 1 hjl hjl 64 Jan 16 16:49 14 -> /export/build/gnu/tools-build/x86_64-linux-toolchain/build-x86_64-linux/binutils/src/binutils-build/ld/tmpdir/stTLiXpO
lrwx------ 1 hjl hjl 64 Jan 16 16:49 15 -> /export/build/gnu/tools-build/x86_64-linux-toolchain/build-x86_64-linux/binutils/src/binutils-build/ld/tmpdir/stTLiXpO
l-wx------ 1 hjl hjl 64 Jan 16 16:49 2 -> pipe:[14151430]
lr-x------ 1 hjl hjl 64 Jan 16 16:49 3 -> pipe:[13933216]
l-wx------ 1 hjl hjl 64 Jan 16 16:49 4 -> pipe:[13933216]
lr-x------ 1 hjl hjl 64 Jan 16 16:49 5 -> pipe:[13990857]
l-wx------ 1 hjl hjl 64 Jan 16 16:49 6 -> pipe:[13990857]
lr-x------ 1 hjl hjl 64 Jan 16 16:49 7 -> /export/build/gnu/tools-build/x86_64-linux-toolchain/build-x86_64-linux/binutils/src/binutils-build/ld/tmpdir/libpr23460.a
lr-x------ 1 hjl hjl 64 Jan 16 16:49 8 -> /export/build/gnu/tools-build/x86_64-linux-toolchain/build-x86_64-linux/binutils/src/binutils-build/ld/tmpdir/pr23460a.o
lr-x------ 1 hjl hjl 64 Jan 16 16:49 9 -> /export/build/gnu/tools-build/x86_64-linux-toolchain/build-x86_64-linux/binutils/src/binutils-build/ld/tmpdir/pr23460b.o

	PR binutils/23460
	* testsuite/ld-plugin/lto.exp: Increase the max number of open
	files to 20 for PR binutils/23460 test.