Literal function name call fails

[RossPatterson]

This issue started as adesutherland/CMS-370-BREXX#61, but the bug is original to bREXX, not peculiar to the CMS370 port. Note that although the original report mentions functions, the CALL instruction behaves identically.

Bob Bolch's original 2021-01-02 bug report there follows.

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When the function name in a function call is a literal, the call fails.

ftest                                             
     2 *-* say 'SUBSTR'('NOK',2,2)                
Error 43 running ftest, line 2: Routine not found 
Ready(20043); T=0.01/0.02 10:05:04  

t ftest exec                                      

/* */                                             
say 'SUBSTR'('NOK',2,2)                           
                                                  
Ready; T=0.01/0.01 10:05:12 

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On [VM/SP et al.] REXX:

ftest                         
OK                            
Ready; T=0.01/0.01 10:05:41