rec: Add endbr64 instructions in the right spots for OpenBSD/amd64

[omoerbeek]

OpenBSD has IBT (aka control flow enforcement) on amd64, for processor models that support it. But it turns out that the existing endbr64 instructions in the boost assembly for context switching are not enough: there needs to be endbr64 instructions right after the jump_fcontext() call, as indirect branches end up there.

Short description

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[coveralls]

Pull Request Test Coverage Report for Build 6690951893

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• 18 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.3%) to 57.697%

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Files with Coverage Reduction	New Missed Lines	%
modules/lmdbbackend/lmdbbackend.cc	2	72.32%
pdns/misc.cc	2	62.8%
pdns/tcpiohandler.cc	2	66.65%
pdns/recursordist/test-syncres_cc1.cc	12	88.87%

Totals
Change from base Build 6690810486:	0.3%
Covered Lines:	105820
Relevant Lines:	152764

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💛 - Coveralls

[rgacogne]

With the caveat that I don't the right hardware to test this, the changes look OK to me. Nice work!