libjsig is being compiled without optimization

It appears libjsig is being compiled without optimization. The reason as to why that is seems rather unclear. It might have been an oversight as with JDK-8210647 (libsaproc). We should compile it with some reasonable opt level.

From a JDK 12 build today:
/usr/bin/gcc -I/disk/openjdk/upstream-sources/openjdk-head-2/build/linux-x86_64-normal-server-release/support/modules_include/java.base -I/disk/openjdk/upstream-sources/openjdk-head-2/build/linux-x86_64-normal-server-release/support/modules_include/java.base/linux -I/disk/openjdk/upstream-sources/openjdk-head-2/src/java.base/share/native/libjava -I/disk/openjdk/upstream-sources/openjdk-head-2/src/java.base/unix/native/libjava -I/disk/openjdk/upstream-sources/openjdk-head-2/src/hotspot/share/include -I/disk/openjdk/upstream-sources/openjdk-head-2/src/hotspot/os/posix/include -pipe -D_GNU_SOURCE -D_REENTRANT -D_LARGEFILE64_SOURCE -DLINUX -DNDEBUG -Wall -Wextra -Wno-unused -Wno-unused-parameter -Wformat=2 -fno-strict-aliasing -m64 -D_LITTLE_ENDIAN -DARCH='"amd64"' -Damd64 -D_LP64=1 -fno-omit-frame-pointer -fno-delete-null-pointer-checks -fno-lifetime-dse -fPIC -fvisibility=hidden -I/disk/openjdk/upstream-sources/openjdk-head-2/src/java.base/unix/native/libjsig -I/disk/openjdk/upstream-sources/openjdk-head-2/build/linux-x86_64-normal-server-release/support/headers/java.base -g -DTHIS_FILE='""' -c -MMD -MF /disk/openjdk/upstream-sources/openjdk-head-2/build/linux-x86_64-normal-server-release/support/native/java.base/libjsig/jsig.d -o /disk/openjdk/upstream-sources/openjdk-head-2/build/linux-x86_64-normal-server-release/support/native/java.base/libjsig/jsig.o /disk/openjdk/upstream-sources/openjdk-head-2/src/java.base/unix/native/libjsig/jsig.c