Simplify make_seed and undo 7b44eb now that register shifts are safe

This needs to be tested more on 32-bit machines

As per discussion: https://github.com/JuliaLang/julia/commit/7b44ebe60f3e0a5e147f3124aa6ea813edadcfa0
fix missing OPENLIBM_FLAGS for make install

this was breaking cross compiles
Merge pull request #6059 from tkelman/patch-2

fix missing OPENLIBM_FLAGS for make install
mention that perf.h is generated in perf.c

https://github.com/JuliaLang/julia/issues/6054

This has come up a couple of times now.
Changed selection of shift-and-invert mode in eigs. Added test for inverse iteration. Docs updated.
Change default of sigma to nothing.
Only non-empty sigma selects shift and invert mode (implies which=LM). Test updated.
Update NEWS.md
Document eye(A) when A is a dense/sparse matrix
New semantics of sigma in eigs(A, sigma)

- `sigma==nothing` specifies forward iteration (ARPACK `iparam(7)=1`), and a numeric real or complex sigma specifies inverse iteration with level shift (`iparam(7)=3` or 4 depending on `complexOP`)
add new vecnorm(x, p), generalizing and replacing normfro; also fix #6056
don't make a copy when hashing substrings
Pkg: pull requests no longer take '?expand=1'

This used to open up to the pull request but no longer does. Now it's better to just show the user the diff and let them click the "create pull request" button on that page.
Merge pull request #6057 from stevengj/vecnorm

add new vecnorm(x, p), generalizing and replacing normfro
Update documentation of eigs

- Specifies _all_ the valid values of the `which` keyword in a table.
- Updates `sigma` to its new default and semantics.
- Clarify interaction of `sigma` and `which` keywords via the type of linear operator that is constructed and passed to ARPACK.
Merge pull request #6060 from stevengj/substring_hash

don't make a copy when hashing substrings
Merge pull request #6023 from jperla/fixmakeseed

Simplify make_seed and undo 7b44eb now that register shifts are safe
Merge pull request #6053 from JuliaLang/pr/5776

Default to shift-and-invert mode for eigs for numeric sigma
Bump openlibm
Fix SEH return val; reset interp on stackoverflow

Allows use of SEH continuation instead of throwing directly.

close #5398 #6021 #6034