Bump build rootfs image

I bumped this in JuliaCI#345, probably for good reason

Author: Zentrik

pipelines/main/platforms/build_linux.pgo_lto.arches

@@ -1,5 +1,5 @@
 # ROOTFS_IMAGE_NAME  TRIPLET                          ARCH     ARCH_ROOTFS    USE_JULIA_PGO_LTO_BOLT    MAKE_FLAGS    TIMEOUT      ROOTFS_TAG   ROOTFS_HASH
-llvm_passes          x86_64-linux-gnu-pgo-lto-bolt    x86_64   x86_64         PGO_LTO_BOLT              .             .            v6.00        0badf5d8794e21a03ac2ff2d46c1ab538ac02571
+llvm_passes          x86_64-linux-gnu-pgo-lto-bolt    x86_64   x86_64         PGO_LTO_BOLT              .             .            v7.6         ebdfa2d40c0e27842cccbfdd43710144f9adf9d8
 
 # These special lines allow us to embed default values for the columns above.
 # Any column without a default mapping here will simply substitute a `.` to the empty string