Import gcrypt public-key cryptography and implement signature checking.

grub-core/lib/libgcrypt/mpi/i586/README

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+This directory contains mpn functions optimized for Intel Pentium
+processors.
+
+RELEVANT OPTIMIZATION ISSUES
+
+1. Pentium doesn't allocate cache lines on writes, unlike most other modern
+processors.  Since the functions in the mpn class do array writes, we have to
+handle allocating the destination cache lines by reading a word from it in the
+loops, to achieve the best performance.
+
+2. Pairing of memory operations requires that the two issued operations refer
+to different cache banks.  The simplest way to insure this is to read/write
+two words from the same object.  If we make operations on different objects,
+they might or might not be to the same cache bank.
+
+STATUS
+
+1. mpn_lshift and mpn_rshift run at about 6 cycles/limb, but the Pentium
+documentation indicates that they should take only 43/8 = 5.375 cycles/limb,
+or 5 cycles/limb asymptotically.
+
+2. mpn_add_n and mpn_sub_n run at asymptotically 2 cycles/limb.  Due to loop
+overhead and other delays (cache refill?), they run at or near 2.5 cycles/limb.
+
+3. mpn_mul_1, mpn_addmul_1, mpn_submul_1 all run 1 cycle faster than they
+should...