Add sntrup761x25519-sha512 post-quantum key exchange #351

Job	Run time
build (plain linux, 1)	1m 38s
build (multi binary, 1, 1)	1m 2s
build (multi binary, dropbearmulti argv0, 1, 1)	1m 8s
build (client only, no, PROGRAMS=dbclient)	1m 21s
build (server only, no, PROGRAMS=dropbear)	34s
build (bundled libtom, bionic , no writev(), ubuntu-20.04, --enable-bundled-libtom --enable-werro...	43s
build (linux clang, clang)	1m 4s
build (c89, -std=c89 -Wdeclaration-after-statement, 1, --enable-pam, #define DROPBEAR_SNTRUP761 0 )	1m 4s
build (macos 14, macos-14, clang, -Wno-deprecated-declarations -Wno-undef, no, no, ranlib -no_war...	29s
build (macos 15, macos-15, clang, -Wno-deprecated-declarations -Wno-undef, no, no, ranlib -no_war...	34s
build (DEBUG_TRACE, 1, --enable-pam, #define DEBUG_TRACE 5 )	1m 5s
build (nondefault options, 1, --enable-pam)	2m 37s
build (most options disabled, --disable-harden --disable-zlib --disable-openpty --disable-lastlog...	2m 13s
build (no sha1, no, #define DROPBEAR_SHA1_HMAC 0 #define DROPBEAR_RSA_SHA1 0 #define DROPBEAR_DH_...	31s
build (pq, no plain x25519, #define DROPBEAR_CURVE25519 0 )	1m 0s
	17m 3s

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