package/gcc: bump to version 7.4.0

package/gcc: bump to version 7.4.0

Remove upstream patches:

0002-fix-building-on-ppc64.patch
https://github.com/gcc-mirror/gcc/commit/aa65a43516da1d48011ef621ed5988289711d99b

0003-gcc-xtensa-fix-NAND-code-in-xtensa_expand_atomic.patch
https://github.com/gcc-mirror/gcc/commit/8709905244d50b9d81f85d534b21c29eb640078c

0003-xtensa-fix-PR-target-65416.patch
https://github.com/gcc-mirror/gcc/commit/5dcbd70ec0a97505d6153fcbc7ef899b5a2ec7fc

0004-libsanitizer-Use-pre-computed-size.patch
https://github.com/gcc-mirror/gcc/commit/61f38c64c01a15560026115a157b7021ec67bd3b

0005-gcc-xtensa-don-t-force-PIC-for-uclinux-target.patch
https://github.com/gcc-mirror/gcc/commit/6ef0a00dea2564f46a568b8681d8efccb4834cda

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>