amd64/prekern: Add ldscript to DPADD since it affects build output.

Specify NOLIBCSANITIZER in x86 bootloader-like code under sys/arch/

Set NOLIBCSANITIZER for i386 and amd64 specific bootloader-like code.

Disable MKSANITIZER

Use ldscript from src to fix build.sh build

Add a PRNG for the prekern, based on SHA512. The formula is basically:

Y0 = SHA512(entropy-file, 256bit rdseed, 64bit rdtsc)
Yn+1 = SHA512(256bit lowerhalf(Yn), 256bit rdseed, 64bit rdtsc)

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Add a PRNG for the prekern, based on SHA512. The formula is basically:

Y0 = SHA512(entropy-file, 256bit rdseed, 64bit rdtsc)
Yn+1 = SHA512(256bit lowerhalf(Yn), 256bit rdseed, 64bit rdtsc)

On each round, random values are taken from the higher half of Yn. If
rdseed is not available, rdrand is used.

The SHA1 checksum of entropy-file is verified. However, the rndsave_t::data
field is not updated by the prekern, because the area is accessed via the
read-only view we created in locore. I like this design, so it will have
to be updated differently.

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style

Add -Wstrict-prototypes, and fix each warning.

Link libkern in the prekern, and remove redefined functions.

Add the amd64 prekern. It is a kernel relocator used for Kernel ASLR (see
tech-kern@). It works, but is not yet linked to the build system, because
I can't build a distribution right now.